The one everybody knows, and the least informative thing in the chart.
Your sun sign is the constellation the Sun was in front of on your birthday. Because the Sun takes a year to go round, it spends roughly a month in each sign, and the dates barely shift. That is why a sun sign can be worked out from a date alone, and why it is the only part of astrology most people ever meet.
It is also why newspaper horoscopes are so thin. One twelfth of the population shares your sun sign. Anything true of all of them has to be vague enough to be true of anyone.
What it actually describes
The Sun is identity: the part of you that stays constant when the room changes. It is real, it is just one placement out of a dozen. Your Moon governs what you feel, your Mercury how you think, your rising sign how you land on people. A chart that only reports the Sun has thrown away almost all of its information.